Thermally-protective intumescent coating

ABSTRACT

An intumescent coating system and method therefor is disclosed for thermally protecting a substrate having a surface exposed to a flame environment. The coating system of the present invention is provided as having a having a first component which forms a rigid carbonific char foam having toughness and rigidity, and a second component which forms an insulative carbonific char foam having a density about half the density of the rigid carbonific foam to give insulation properties. The two foam components may be separately provided in a laminate-type arrangement as, respectively, an inner coating layer coated and cured on the surface of the substrate to form an inner film layer, and an outer coating layer coated and cured on the inner film layer to form an outer film layer. Alternatively, the two foams may be provided as being formed from components blended to comprise a single coating layer which is coated and cured on the surface of the substrate to form a film layer. In providing two foam components, one protecting the substrate from breakthrough and direct exposure to the flame environment, and the other insulating the substrate to protect it from conductive, radiant, and/or convective heating by the flame environment, the present invention affords a capability to offer flame retardancy at film thicknesses of less than 50 mils.

The Government of the United States has rights in this invention under contract No. A4124-900.

BACKGROUND OF THE INVENTION

The present invention relates to intumescent coatings, such as those used as paints and the like, and to methods utilizing such coatings for fire retardance. For structures fabricated from polymeric materials and the like, flame spread and thermal decomposition generally is accelerated to the point that normal fire fighting response times are inadequate to prevent significant combustion of the structure. Accordingly, intumescent coatings have been employed as paints to form a protective layer on the surface of such structures. As the name implies, intumescent coatings bubble when exposed to flames and produce an insulative layer of char and char foam.

The relatively low tendency for elemental carbon to oxidize has led to its incorporation into intumescent coatings. In this regard, highly-flammable substrates can be thermally-protected by application of a surface coating having ingredients catalyzed to be pyrolized into a carbonific char. Thus, the coating is converted from an oxidizable composition into elemental carbon. Broadly, such coatings comprise a polyhydric organic compound, an acid forming catalyst, and a blowing agent which intumesces the carbonific char formed from the acid-catalyzed pyrolysis of the polyhydric compound into a carbonific char foam having a relatively low the conductivity.

On its own, a pure carbon foam would not be expected to provide optimal thermal insulation. Consequently, the carbon foam often is supplemented with inorganic additives. Some inorganics, e.g., titanium dioxide and zinc oxide, provide nucleating sites for gas formation, resulting in fine cell, low density foam. Inasmuch as heat conduction through a gas can be several orders of magnitude lower than through a solid, a low density foam provides insulation improved over a higher density foam, especially when coated on a heat-conducting substrate such as a metal or a carbon-fiber composite. Additionally, several inorganics, including zirconium salts, borates, phosphates, and titanium dioxide, can contribute to the formation of a refractory layer over the top of the foam layer. The refractory layer advantageously provides an inert, highly infrared reflective layer, which can contribute significantly to the insulative properties of the foam. The infrared reflectance of the char also may be improved by the addition of inorganics having low emissivities such as titanium dioxide, zirconium dioxide, phosphate and antimonate glass. Other inorganics such as silica microballoons and silicone resins may be added to decrease heat conduction through the foam.

To form intumescent coatings having good integrity and offering resistance to high humidity, water-insoluble intumescent or char-forming agents, including selective salts of nitro aromatic amine compounds such as 4,4'-dinitrosulfanilimide, have been blended with epoxy-polysulfide or epoxy-cholorsulfonated polyethylene binder systems. Although such formulations are efficient intumescents, their efficiency is not optimized because the nitro aromatic intumescent species produces an exothermic char-forming reaction. To counter this exothermic effect, ablatives and endothermic fillers, including zinc borate and hydrated endothermic fillers such as aluminum hydroxide pigments, are beneficial.

The intumescent coatings heretofore known in the art have been used successfully to effectively reduce flame spread and to protect substrates frown thermally-induced mechanical/chemical decomposition. Although these coatings have demonstrated significant fire retardant properties, they generally must be used at thicknesses, e.g., >200 mils, which precludes their use in applications where weight requirements are a consideration. Moreover, conventional coatings are not formulated to withstand exposure to the severe environments common in marine applications and the like, and often have ingredients which leach out after extended immersion in seawater. For certain applications, the intumescent reaction temperature of conventional formulations may be at or near the service temperature. Accordingly, there has existed and remains a substantial need for intumescent coating systems which not only will afford the requisite thermal insulation at minimum coating thicknesses and at elevated service temperatures, but which also will retain an insulative capability even after exposure to harsh marine environments and the like.

BROAD STATEMENT OF THE INVENTION

The present invention is addressed to thermally-protective, intumescent coating systems and methods therefor which provide coatings having outstanding fire protection properties at minimal film thicknesses of less than 50 mils. Advantageously, such coatings are able to withstand typical sea exposure with no apparent loss in heat barrier properties, exhibit a strong adherence to hull materials, and are coatable with marine antifouling paint.

In this regard, it has been discovered that, owing to the severe thermal and convective environment developed during fire conditions, there exists a critical region for obtaining toughness, stability, and prevention of char breakthrough of an intumescent coating or paint. By "breakthrough," it is meant the point at which the char or char foam formed from the decomposition of the coating layer is broken, separated, or otherwise affected by the fire to expose the substrate directly to the flame environment. The present invention therefore involves providing the coating as having a first component which forms a rigid carbonific char foam having superior toughness and rigidity, and a second component which forms an insulative carbonific char foam having a density about half of the density of the rigid carbonific foam to give superior insulation properties. In providing two foam components, one protecting the substrate from breakthrough and direct exposure to the flame environment, and the other insulating the substrate to protect it from conductive, radiant, and/or convective heating by the flame environment, the present invention affords a capability to offer superior fire protection at film thicknesses of less than 50 mils.

It will be appreciated that the two foam components may be separately provided in a laminate-type arrangement as, respectively, an inner coating layer coated and cured on a surface of the substrate to form an inner film layer, and an outer coating layer coated and cured on the inner film layer to form an outer film layer. It is preferred that the inner film layer is selected as intumescently decomposing to form the rigid char foam component having superior toughness and rigidity, with the outer film layer selected to form the insulative char foam component having superior insulation properties. With such an arrangement, the inner film layer is made to remain intact until the outer film layer intumescently decomposes into the insulative char foam, which foam insulates the substrate from the heat of the flame environment, but which may be eventually broken or otherwise degraded by the aggressive nature of the same. However, at the locations whereat the outer insulative char foam layer has broken through to expose the inner film layer directly to the flame environment, the inner film layer advantageously then decomposes to form the rigid char foam which protects the substrate from direct expose to the flame environment and thereby provides an additional degree of fire protection.

Alternatively, the two foams may be provided as being formed from components blended to comprise a single coating layer which is coated and cured on the surface of the substrate to form a film layer. For forming the rigid char foam, a first component is provided in the coating layer which component is curable to form a first portion of the film layer which portion is thermally decomposable to form a char which intumesces to form the rigid char foam. For forming the insulative char foam, a second component is provided in the coating layer which component is curable to form a second portion of the film layer which portion is blended with the first portion. The second portion of the film layer is thermally decomposable to form a char which char intumesces to form the insulative char foam which foam is blended with the rigid char foam. Again, the rigid char foam component protects the substrate from breakthrough and the direct exposure to the flame environment, with the insulative char foam component protecting the substrate from conductive, radiant, and/or convective heating by the flame environment.

It is, therefore, an aspect of the present invention to provide a coating system for thermally protecting a substrate having a surface exposed to a flame environment. The coating system involves a first coating layer coated on the surface of the substrate and cured thereon to form a first film layer which layer is thermally decomposable upon exposure to the flame environment to form a first carbonific char which char intumesces to form a rigid carbonific char foam. A second coating layer is coated on the first film layer and cured thereon to form a second film layer which layer is thermally decomposable upon exposure to the flame environment to form a second carbonific char which char intumesces to form an insulative carbonific char foam having a density about half the density of the rigid carbonific char foam. Advantageously, the rigid carbonific char foam is made to form where the insulative carbonific char foam has broken through to expose the first film layer to the flame environment.

Another aspect of the present invention is to provide a method for thermally protecting a substrate having a surface exposed to a flame environment. The method involves the steps of coating the surface of the substrate with a first coating layer, and then curing the first coating layer thereon to form a first film layer which layer is thermally decomposable upon exposure to the flame environment to form a first carbonific char which char intumesces to form a rigid carbonific char foam. A second coating layer is coated on the first film layer, and then is cured thereon to form a second film layer which layer is thermally decomposable upon exposure to the flame environment to form a second carbonific char which char intumesces to form an insulative carbonific char foam having a density about half the density height of the rigid carbonific char foam. Advantageously, the rigid carbonific char foam is made to form where the insulative carbonific char foam has broken through to expose the first film layer to the flame environment.

Yet another aspect of the present invention is a coating composition for thermally protecting a substrate having a surface exposed to a flame environment on which the composition is coated and cured to form a film layer. The composition is formulated from a blend of a first and a second component. The first component is curable to form a first portion of the film layer, the first portion being thermally decomposable to form a first carbonific char which char intumesces to form a rigid carbonific char foam. The second component is curable to form a second portion of the film layer which portion is blended with the first portion. The second portion is thermally decomposable to form a second carbonific char which char intumesces to form an insulative carbonific char foam blended with the rigid carbonific char and having a density about half the density of the rigid carbonific char foam.

Still another aspect of the present invention is the provision of a method for thermally protecting a substrate having a surface exposed to a flame environment. The method involves the step of coating the surface of the substrate with a coating layer formulated from a blend of a first component and a second component. The first component is curable to form a first portion of a film layer, which portion is thermally decomposable to form a first carbonific char which char intumesces to form a rigid carbonific char foam. The second component is curable to form a second portion of the film layer which portion is blended with the first portion and is thermally decomposable to form a second carbonific char which intumesces to form an insulative carbonific char foam blended with the rigid carbonific char. The insulative char foam is provided to have a density of about half the density of the rigid carbonific char foam. The coating layer is cured on the surface of the substrate to form the film layer.

An advantage of the present invention includes a thermally-protective, intumescent coating system having improved insulation properties at minimal film thickness of about 50 mils, and which is suitable for marine and other applications. Another advantage is the ability to provide laminar coating composition optimizing the fire protection properties of each of the laminar. These and other advantages will be readily apparent to those skilled in the art based upon the disclosure contained herein.

DETAILED DESCRIPTION OF THE INVENTION

The intumescent coating system and method of the present invention is provided as having a first component which is curable to form a film layer, and a second component which also is curable to form a film layer. By "curable," it is meant that the first and second components each polymerizes, cross-links, vulcanizes, or otherwise reacts to form a thermoplastic or thermosetting polymeric structure. In accordance with the precepts of the invention, the first and second component may be applied to a substrate in a laminate-type arrangement as, respectively, an inner coating layer coated and cured on a surface of the substrate to form an inner film layer, and an outer coating layer coated and cured on the inner film layer to form an outer film layer. Alternatively, the two components may be provided as blend which blend comprises a single coating layer which is coated and cured on the surface of the substrate to form a film layer. In either arrangement, the first component is selected as forming a film layer or a portion thereof which thermally decomposes upon exposure to a flame environment to form a carbonific char which intumesces to form a rigid carbonific char foam which is effective to protect the substrate from breakthrough and direct exposure to the flame environment. The second component is selected relative to the first component as forming a film layer or a portion thereof which thermally decomposes upon exposure to a flame environment to form a carbonific char which intumesces to form an insulative carbonific char foam which is effective to insulate the substrate and protect it from conductive, radiant, and/or convective heat transfer from the flame environment.

When employed in a laminate-type arrangement, it has been observed that the inner film layer formed from the curing of the first component is provided to remain intact until the outer film layer formed from the curing of the second component intumescently decomposes into an insulative char foam. Although the insulative char foam insulates the substrate from the heat of the flame environment, it is necessarily provided as having an open cellular morphology which may be eventually broken or otherwise degraded by the aggressive nature of the flames. In this regard, however, the inner and outer layers function synergistically in that at the locations where the outer insulative char foam layer has broken through to expose the inner film layer directly to the flame environment, the inner film layer advantageously then decomposes to form the rigid char foam which protects the substrate from direct expose to the flame environment and thereby provides an additional degree of fire protection. Alternatively, when the first and second components of the invention are applied as a blended layer, which layer thermally decomposes and intumesces to form a foam blend, the insulative char foam portion of the foam blend again is seen as protecting the substrate from damage from conductive, radiant, and/or convective heat transfer from the flame environment. Likewise, the rigid char foam portion of the foam blend again is seen as being effective to strengthen the foam blend and to thereby protect the substrate from the direct exposure to the flame environment which would attend any breakthrough of the foam.

As to the physical properties which the foams of the first and second component of the invention are selected to exhibit, it is preferred that the cured film of the first component intumesces into a rigid or hard carbonific char foam having a density about twice that of the insulative or soft carbonific char foam of the cured film of the second component. In this regard it will be appreciated that, as heat conduction through a gas can be several orders of magnitude lower than through a solid, the lower density foam of the second component will exhibit a thermal conductivity correspondingly lower than that of the higher density foam of the first component. However, as the foam of the cured film of the first component is twice as dense, it will be correspondingly stronger, harder, and more rigid than the necessarily softer foam of the cured film of the second component.

Quantitatively, its is preferred that the rigid carbonific char foam of the first component is selected as having a density of between about 0.5 g/cm³ and less than about 0.10 g/cm³, with the insulative carbonific char foam being selected as having a density of between greater than about 0.01 g/cm³ and less than about 0.5 g/cm³. The rigid foam also will be seen as having a foam expansion height about half the expansion height of the insulative foam. In this regard, a 0.254 mm (10 mil) thick layer of the cured film of the first component has been observed to intumesce into a foam having an expansion height of 2.54 cm (1-inch), while a 0.254 mm (10 mil) thick layer of the cured film of the second component has been observed to intumesce into a foam having an expansion height of 5.08 cm (2-inch). As measured by ASTMD 1415-56T, the rigid foam of the first component is correspondingly harder as having a penetration depth of only 0.3 mm (11.8 mil) as compared to the relatively softer foam of the second component having a penetration depth of 0.7 mm (27.6 mil).

Considering the preferred formulation of the second component, the cured film thereof which thermally decomposes and intumesces into the insulative carbonific char foam component of the present invention, the component is formulated as comprising a combination of ingredients which are curable but reactive upon subsequent thermal exposure to form an incombustible residue which is expandable to a carbonific cellular foam. These ingredients, however, are selected to melt, react or decompose in a particular sequence to develop a desired viscosity profile, as well as a desired time-temperature history which is the sum of the exothermic or endothermic reactions occurring inside the coating, and of the net heat transfer into and out of the intumescing coating. Broadly, the second component of the present invention comprises a polymeric binder or vehicle, a char promoter, a dehydrating agent, a spumific or blowing agent, and optionally, pigments and solvents.

Preferably, the binder or vehicle for carrying the intumescent ingredient mixture is curable and comprises an epoxy-polysulfide system or, alternatively, an epoxycholorsulfonated polyethylene system, an aminoplastic system such as urea-melamine-formaldehyde, or a siloxane polymer such as a polysiloxane, a polysilarylene, or a derivative thereof, such as, for example, Siloxirane™ 2433 manufactured by Advanced Polymer Sciences, Inc., of Avon, Ohio. If an epoxy-based binder system is employed, the catalyst for the epoxy resin, preferably a polysulfide, polyamine, polyamide, or the like, may be provided as a B-stage component for addition to the A-stage resin prior to application. As the name suggests, the binder, typically in conjunction with a solvent, wets and holds together the ingredients in the coating formulation. Before cure, the binder provides for homogeneous film-forming. After cure, the binder imparts mechanical integrity to make the coating durable. The binder also assists in sealing the char for improved foaming and can contribute to the amount of char developed.

The char developed in the intumescent coating formulation of the present invention is formed by the acid-catalyzed dehydration of a polyhydric char promoter to yield a carbon char and water. For purposes of the formulation of the present invention, the polyhydric substance is a polyol, with pentaerythritol, dipentaerythritol, and derivatives and mixtures thereof being preferred. Alternatively, other char promoters, such as sugars, polyhydric phenols, or starches, may be substituted.

The polyhydric char promoter of the subject formulation is selected to contain hydroxyl functional groups which are subject to a dehydration reaction. Accordingly, a dehydrating agent, preferably an acid or an acid precursor such as an ammonium polyphosphate, a magnesium phosphate, a phosphite, or an organophosphate ester which is thermally decomposable to form an acid catalyst such as phosphoric acid, is included to dehydrate the char promoter to form water and an intermediate species such as an ester. Advantageously, the intermediate species is readily thermally decomposable to form a relatively incombustible carbonific char which may made somewhat self-extinguishing via the liberation of carbon dioxide during the dehydration. Moreover, the latent heat of evaporation of the water condensed by the dehydration of the char promoter endothermically contributes to the insulative effects of the coating formulation.

The char developed during the acid-catalyzed dehydration of the char promoter is expanded into a carbonific char foam having a relatively low thermal conductivity which, together with the endothermic decomposition and dehydration reactions, effects the insulative efficiency of the coating. To expand the char, a blowing agent or spumific is employed for its decomposition at a desired temperature and attendant liberation of a nonflammable gas such as nitrogen. For purposes of the present invention, a blowing agent such a dicyandiamide or an azodicarbonamide is preferred, although another blowing agent such as a melamine, a guanidine, a glycine, a urea, or an organohalophosphate ester derivative, or a chlorinated or halogenated organic material such as a chlorinated paraffin, may be substituted. Of utmost importance to the formation of a thick, uniform foam, however, is that the blowing agent decomposes at a temperature higher than the decomposition temperatures of both the intermediate species and the dehydrating agent.

As to optional additives, the toughness of the carbon foam may be improved through the use of thermally-stable aminoplast resins such as urea-melamine-formaldehyde resins which form a tough matrix for the cellular mass. Similarly, glass-forming materials such as borates, silicates, mica, glass fibers and the like optionally may be added as inert, char-reinforcing agents. A zinc oxide additive may be employed to reduce the amount of smoke generated during the intumescent reaction. Additionally, the viscosity of the formulation may be thinned with a solvent such as methyl ethyl ketone, methyl isobutyl ketone, a naphtha, or a mixture thereof to facilitate spray application.

Considering now the preferred formulation of the first component, such formulation may be functionally characterized as curable to form a film which film thermally decomposes and intumesces into the hard or rigid carbonific char of the present invention selected as having a structural integrity sufficient to withstand the turbulent convective currents generated in an active fire or flame environment. In this regard, a coating marked under the name FIREX™ by the Pfizer Co. of New York, N.Y., has been found to meet the necessary functional requirements. The composition of the FIREX^(TM) brand coating may be broadly described as a two-part, epoxy resin system comprising a first component of a filled polysulfide polymer and a second component of a filled liquid epoxy resin.

Advantageously, the coating method and system of the present invention, wherein a first component is provided to form a hard, rigid foam and a second component is provided for form a soft, insulative foam, may be adapted for a variety of applications. For example, depending upon the application contemplated and whether insulation value or coating weight is the overwhelming consideration, the practitioner may utilize two coating layers of the second component over a primer coat layer of the first component. A topcoat of the first component also may be employed over the second component coating layers. In such an arrangement, it has been observed that the topcoat layer, the cured film of which forms a hard or rigid char foam, protects the developing foam of the inner layers of the second component from breakthrough. In this way, the insulation and char stability of each layer in the coating laminate is optimized to achieve improved fire protection capabilities. Additionally, and as aforementioned, the first and second components of the present invention may be provided in a blended form. Such a blend is attractive in that acceptable fire protection may be achieved with fewer coats and, accordingly, a reduced overall coating thickness.

As the experimental data will show, the bi-component coating system of the present invention provides superior fire protection as compared to a single component coating when applied to, for example, marine hull materials such as DURA. DURA is an advanced special hull treatment material, proprietary to the U.S. Navy, which is described in The Journal of the Acoustical Society of America, Vol. 77, No. 3, pp. 1229-1238 (1985), to comprise a polyurethane formed from a toluene diisocyanate and a polytetramethylene glycol. Indeed, in addition to providing superior fire protection, the components of the coating system of the invention have been found to adhere strongly to marine hull materials such as DURA and the like, to be coatable with antifouling paint, and to be capable of withstanding typical sea exposure with no apparent loss in insulative properties.

Additionally, the coating system of the present invention will find application in the general fire protection of homes, commercial buildings, and the like. In this regard, the inventive system may be applied as a coating to ceiling tiles which typically are supported with a metal grid, tray, or runner assembly, or with a tongue-in-groove, interlocking arrangement. During fires, it has been observed that the fibrous material from which the ceiling tiles are formed undergoes a degree of shrinkage, which shrinkage loosens the tiles and results in their falling from the ceiling. The expansion of the coating system of the invention, however, accommodates for the shrinkage of the tiles. The tiles therefore are made to remain secure in their support to lessen the chances that a occupant of the home or building will be struck by a falling tile. For such applications, it has been found that a polysiloxane or a derivative thereof is preferred for the binder component of the system as promoting a better adhesion with ceiling tile materials.

The examples to follow are illustrative of the precepts of the present invention but should not be construed in a limiting sense. All percentages and proportions are by weight, unless otherwise expressly indicated.

EXAMPLES

A pilot plant batch of an intumescent coating composition, Formula No. 45852-232, the cured film of which forms an insulative char foam according to the precepts of the present invention, was prepared as a two-part system having a weight per gallon of 10.982 lbs. (6.358 kg/1), a total solids content of 76%, and a composition as follows:

                                      TABLE 1                                      __________________________________________________________________________     Ingredients   Description                                                                               Weight-%                                                                              Volume-%                                       __________________________________________________________________________     Component A                                                                    Dipentaerythritol                                                                            char promoter                                                                             13.56  13.14                                          Ammonium Polyphosphate.sup.1                                                                 dehydrating agent                                                                         27.13  19.96                                          Azodicarbonamide.sup.2                                                                       spumific    1.74   1.39                                          Polysulfide.sup.3                                                                            binder     12.74  13.22                                          Epoxy Resin.sup.4                                                                            binder     12.74   9.70                                          Magnesium Phosphate                                                                          acid catalyst                                                                              7.05   4.23                                          Black Iron Oxide                                                                             pigment     0.54   0.15                                          Methyl Isobutyl Ketone                                                                       solvent     9.65  15.86                                          High Flash Naphtha                                                                           solvent    14.48  21.87                                          Total Component A        99.63  99.52                                          Component B                                                                    Primary Amine epoxy resin catalyst                                                                       0.37   0.48                                          Total Component B         0.37   0.48                                          __________________________________________________________________________      .sup.1 PHOSCHECK P30 ™ (Monsanto, St. Louis, MO)                            .sup.2 CELOGEN AZ ™ (Uniroyal, Middleberg, CN)                              .sup.3 THIOKOL LP3 ™ (Morton International, Chicago, IL)                    .sup.4 EPI-REZ 5183 ™ (Rohne Poulene, Louisville, KY)                       .sup.5 EH-330 ™ (Morton International, Chicago, IL)                   

Prior to the addition of the component B catalyst, component A was mixed thoroughly with a power mixer and was shaken for 30 minutes. The component B catalyst then was added to the thoroughly mixed Component A at a weight ratio (A/B) of 100/0.37. To facilitate the addition, the mixed Component A was placed under slow agitation using a power mixer with Component B being slowly added. The agitation was continued for 5 minutes. The pot life of the catalyzed mixer was found to be about 8-10 hours depending upon the ambient temperature.

EXAMPLE 1

To validate the precepts of the present invention, the No. 232 formulation prepared above was used as a coating layer in combination with the commercial FIREX™ formulation selected as thermally decomposing to form the hard or rigid carbonific char of the present invention. In this regard, DURA panels were coated with various layers of the No. 232 and FIREX™ formulations, with a single FIREX™ layer coating being used as a control. The compositions of the coating layers employed were as follows:

                  TABLE 2                                                          ______________________________________                                         Coating Layer                                                                              Composition       Weight-%                                         ______________________________________                                         FIREX ™  Component A.sup.1 57.0                                                         Component B.sup.2 43.0                                             Formula No. 232                                                                            Component A       99.63                                                        Component B        0.37                                            Topcoat Blend                                                                              FIREX ™ (as above)                                                                            75.00                                                        Formula No. 232 (as above)                                                                       25.00                                            ______________________________________                                    

Each of the DURA test panels was power buffed with a disc sander using 40-grit paper. The resulting appearance was a smooth, dull gray finish. To the sanded DURA panels were applied various layers of the coating compositions of Table 2. All coatings were applied using a WAGNER™ Heavy Duty Power Painter airless sprayer equipped with a 0.6-mm nozzle. Samples were coated as listed in Table 3 according to the coating and cure schedule described in Table 4.

                  TABLE 3                                                          ______________________________________                                                         Sample No.                                                                     (Dry Film Thickness, mils)                                     Coating Layer     61 (control)                                                                             61-1   61-2 61-3                                   ______________________________________                                         FIREX ™ primer (70% in                                                                        50        14.4   11.2 11.3                                   MEK).sup.1                                                                     Formula No. 232 (1st coat).sup.2                                                                           10.1    9.1  8.9                                   Formula No. 232 (2nd coat).sup.2                                                                           11.4   12.4 12.9                                   FIREX ™ topcoat (67% in         15.2                                        MEK).sup.3                                                                     FIREX ™/No. 232 blend                 9.0                                   (75/25).sup.4                                                                  Total Dry Film Thickness                                                                         50        35.9   47.9 42.1                                   ______________________________________                                          .sup.1 Material received at 80% and thinned for spraying by addition of        10% methyl ethyl ketone by weight.                                             .sup.2 74% solids as manufactured                                              .sup.3 Material received at 80% solids and thinned for spraying by             addition of 14% methyl ethyl ketone by weight to achieve a uniform, wet        coating. Although the material sprayed easily and exhibited the desired        wet film properties, some solvent popping was observed which resulted in       an occasional small pinhole. Accordingly, slightly less solvent, e.g.,         8-10%, is indicated.                                                           .sup.4 FIREX ™ received at 80% solids and thinned for spraying by           addition of 13% methyl ethyl ketone. Formula No. 232, 74% solids as            manufactured, was blended with the FIREX ™ (75/25 FIREX ™/No. 232 b      weight).                                                                 

                  TABLE 4                                                          ______________________________________                                         Day  Preparation                                                               ______________________________________                                         1    Apply FIREX ™ primer (all Samples);                                         Retain panels in spray area for 4 hours;                                       Place panels in controlled environment                                         (75° F., well ventilated); and                                          Age panels 24 hours.                                                      2    Apply 1st coat Formula No. 232 (Samples 61-1, 61-2, 61-3);                     Retain panels in spray area for 4 hours;                                       Place panels in controlled environment                                         (75° F., well ventilated); and                                          Age panels 72 hours..sup.1                                                5    Apply 2nd coat Formula No. 232 (Samples 61-1, 61-2, 61-3);                     Retain panels in spray area for 4 hours;                                       Place panels in controlled environment                                         (75° F., well ventilated); and                                          Age panels 48 hours..sup.1                                                7    Apply topcoats according to Table 3 (Samples 61-2, 61-3);                      Retain panels in spray area for 4 hours; and                                   Place panels in controlled environment                                         (75° F., well ventilated).                                         9    Remove panels for adhesion testing.                                       ______________________________________                                          .sup.1 Although aging times of 48 and 72 hours are noted, and while longe      aging times are acceptable provided intercoat adhesion is attained, it ha      been demonstrated that an aging time of 24 hours at 75°  F. and         less than 50% Relative Humidity is sufficient.                           

In summary, no unexpected or unusual conditions were encountered in the preparation of the test panels. All films were applied as wet coatings, and the cured films exhibited a good appearance.

The adhesion of each coat applied to each of the DURA panels was evaluated using a tape pull as specified in A.S.T.M. 3359. The results were as follows:

                  TABLE 5                                                          ______________________________________                                                            Sample No.                                                  Coating Layer        61-1    61-2   61-3                                       ______________________________________                                         FIREX ™ primer    5A      5A     5A                                         (Adhesion after 24 hours).sup.1                                                Formula No. 232 (1st coat)                                                                          4A      4A     4A                                         (Adhesion after 72 hours).sup.1                                                Formual No. 232 (2nd coat)                                                                          5A      5A     5A                                         (Adhesion after 48 hours).sup.1                                                FIREX ™ topcoat.sup.1     5A                                                FIREX ™/No. 232 blend (75/25)    5A                                         (Adhesion after 48 hours).sup.1                                                ______________________________________                                          .sup.1 Adhesion evaluated by tape pull procedure specified in A.S.T.M.         3359. Ratings of 4A and 5A are considered to be excellent.               

In summary, the intercoat adhesion between each coat in the coatings, as well as the overall adhesion of the entire coating laminate, was found to be excellent.

EXAMPLE 2

To validate the fire protection afforded according to the precepts of the present invention, the thermal responses of the coating systems described in Table 3 were evaluated as measured by the fire resistance of the substrate. The DURA test panels (12-inch by 12-inch, 0.3 m by 0.3 m) prepared in accordance with the coating protocol set forth in Tables and 4, were quartered for burn testing, as was the control specimen, Sample No. 61, which was prepared as having only a single FIREX™ coating layer of a dry film thickness of 50 mils.

The burn testing was conducted using a small scale fire test apparatus. The apparatus consisted of a Fisher burner (Model No. 03-900) having a 1-inch (2.54 cm) diameter diffuser cap (Model No. 03-900-10). The burner was operated with its air vents fully opened, and the gas input rate was adjusted to generate a 1/8-inch (0.3175 cm) high flame cone immediately above the cap of the burner.

Each of the 4-inch by 4-inch (10.16 cm by 10.16 cm) test specimens was centered above the burner at a 45° angle and at a nominal distance of 2.5 inches (6.35 cm) from the center of the specimen to the top of the burner. The burner then was adjusted to achieve the 8-inch (0.3175 cm) flame cone which was maintained under the specimen for 15 to 20 minutes. The essentially convective heat flux into the specimen was estimated at 30,000 Btu/hr/ft² (3.51×10³ kJ/hr/m²). The following results were observed:

                  TABLE 6                                                          ______________________________________                                         Sample                                                                               Time Over Flame                                                          No.   (minutes:seconds)                                                                           Observation                                                 ______________________________________                                         61-1.sup.1                                                                           3:30         Char fully expanded and edges are                                              separated away from FIREX ™                                                 primer.                                                           3:35         FIREX ™ primer slightly                                                     visible around edges of char,                                                  but is not                                                                     involved in fire or char. Primer                                               appears slightly liquid.                                          3:40         Conditions stable, no flaming or                                               dripping.                                                         15:00        No change. Char bubble stable.                                    60:00        No change. Test discontinued.                               61-2.sup.2                                                                           1:00         Large 5-inch (12.7 cm) char                                                    bubbled formed. Minor surface                                                  flaming for 30 seconds.                                           2:00         Char bubble stable.                                               15:00        Char bubble stable.                                               60:00        Char bubble stable. No smoking                                                 or dripping. Entire panel                                                      surface intact.                                                                Test discontinued.                                          61-3.sup.3                                                                           1:00         Some surface flaming. Small bubble                                             formed (about half the size of                                                 bubble formed in Sample No. 61-2).                                2:00         Char bubble stable; flame                                                      extinguished.                                                     10:00        Coating expanded away from                                                     sample at edges where sample was                                               quartered. Some dripping from                                                  bottom edge cut.                                                  12:00        Dripping stopped.                                                 22:00        Smoking from separation on both                                                side edges. Center char bubble                                                 intact.                                                           35:00        Some smoking but no dripping.                                                  Center char bubble is stable.                                     60:00        No change. Entire surface panel                                                intact. Test discontinued.                                  61.sup.4                                                                             5:00         Major cracks in cneter of char                                                 bubble.                                                           5:37         Some flaming from center cracks.                                  9:00         Cracks opening around bottom                                                   perimeter of char bubble and some                                              flaming. Center cracks "healed";                                               no flaming.                                                       11:00        Flaming around char perimeter, but                                             DURA not yet ignited.                                             13:30        Smoking and passive flames                                                     continue.                                                         16:20        Char bubble breaks open.                                          17:40        Active burning.                                                   19:10        DURA involved in fire, major                                                   dripping.                                                         20:30        DURA flaming; test discontinued.                            ______________________________________                                          .sup.1 FIREX ™ primer, 2 coats No. 232, no topcoat                          .sup.2 FIREX ™ primer, 2 coats No. 232, FIREX ™ topcoat                  .sup.3 FIREX ™ primer, 2 coats No. 232, FIREX ™/No. 232 (75/25)          blend topcoat                                                                  .sup.4 FIREX ™ control                                                

The burn test results confirm the precepts of the invention in that optimal fire protection is provided when a bi-component coating system is employed to provide two foam components, one being relatively rigid and hard to protect the substrate from breakthrough and direct exposure to the flame environment, and the other being relatively soft and insulative to protect the substrate from conductive, radiant, and/or convective heating by the flame environment. Although Sample No. 61-2 (FIREX™ topcoat over 2 coats of Formula No. 232 and 1 coat of FIREX™ primer) qualitatively appears to be preferred in maintaining the most integrity overall, i.e., even after 60 minutes there was no breaking in the coating surface, Sample No. 61-1 (2 coats of Formula No. 232 over 1 coat of FIREX™ primer) nevertheless provides fire performance improved over that attainable with the conventional intumescent coating of Sample No. 61 (FIREX™ control). Sample No. 6-3 (FIREX™/No. 232 blend topcoat over 2 coats of Formula No. 232 and 1 coat of FIREX™ primer) also exhibits improved fire protection and thereby validates another aspect of the invention.

EXAMPLE 3

The fire protection afforded according to the precepts of the present invention was again validated using a second burn test procedure designed to more approximate the effects of an aggressive, large-scale fire. This procedure entailed igniting a 2 ft by 2 ft (0.61 m by 0.61 m) pan of heptane beneath a specimen held at a 45° angle to the pan. The thermal responses of 3 ft by 3 ft (0.9 m by 0.91 m) DURA samples prepared in accordance with Tables 3 and 4 were compared with that of a control specimen coated with a 50 mil FIREX™ layer. The essentially radiative heat flux into the specimen was estimated to be 15,000 Btu/hr/ft² (1.76×103 kJ/hr/m²). The fire resistance afforded by the coatings was noted as the time in minutes to the ignition of the substrate, with the following results having been observed:

                  TABLE 7                                                          ______________________________________                                                        Sample No.                                                      Composition      61-1   61-2   61-3 61 (Control)                               ______________________________________                                         FIREX ™ primer                                                                               X      X      X                                               Formula No. 232 (1st coat)                                                                      X      X      X                                               Formula No. 232 (2nd coat)                                                                      X      X      X                                               FIREX ™ topcoat      X                                                      FIREX ™/No. 232             X                                               (75/25) blend topcoat                                                          FIREX ™ Control (50 mil)         X                                          Fire Resistance  17:40  >24.sup.1                                                                             16:23                                                                               7:35                                       (min:sec to failure)                                                           ______________________________________                                          .sup.1 The pan of heptane supported burning for approximately 8 minutes,       but would extinguish and have to be replaced. After 3 pans of heptane, th      fire resistance of Sample No. 612 was recorded as >24 minutes. However, a      the char of Sample No. 612 was observed still to be in excellent               condition, it undoubtedly would have been stable for considerably longer       than 24 minutes.                                                         

These data corroborate the validity of the precepts of the invention in that optimal fire protection again is observed to be provided when a bi-component coating system is employed to provide a rigid or hard foam component to protect the substrate from breakthrough and direct exposure to the flame environment, in conjunction with a soft or insulative foam component to protect the substrate from conductive, radiant, and/or convective heating by the flame environment. Again, although Sample No. 61-2 (FIREX™ topcoat over 2 coats of Formula No. 232 and 1 coat of FIREX™ primer) qualitatively appears to be preferred in maintaining the most integrity overall, Sample No. 61-1 (2 coats of Formula No. 232 over 1 coat of FIREX™ primer) nevertheless provides fire performance improved over that attainable with the conventional intumescent coating of Sample No. 61 (FIREX™ control). Sample No. 6-3 (FIREX™/No. 232 blend topcoat over 2 coats of Formula No. 232 and 1 coat of FIREX™ primer) also exhibits improved fire protection and thereby again validates another aspect of the invention. 

What is claimed:
 1. A coating system for thermally protecting a substrate having a surface exposed to fire conditions comprising:(a) a first coating layer coated on said surface of said substrate and cured thereon to form a first film layer which layer is thermally decomposable upon exposure to said fire conditions to form a first carbonific char which char intumesces to form a first rigid carbonific char foam; and (b) a second coating layer coated on said first film layer and cured thereon to form a second film layer which layer is thermally decomposable upon exposure to said fire conditions to form a second carbonific char which char intumesces to form an insulative carbonific char foam having a density about half the density of said first rigid carbonific char foam, said first rigid carbonific char foam forming where said insulative carbonific char foam has broken through to expose said first film layer to said fire conditions; and wherein said coating system has a thickness of less than 50 mils.
 2. The coating system of claim 1 wherein said insulative carbonific char foam has a density of between about 0.01 g/cm³ and less than about 0.5 g/cm³.
 3. The coating system of claim 1 wherein said second coating layer comprises:a curable binder; a char promoter having hydroxyl groups; a dehydrating agent which is thermally decomposable to form an acid catalyst, said char promoter dehydrating in the presence of said acid catalyst to form an intermediate species which intermediate species is thermally decomposable to form said second carbonific char; and a spumific which is thermally decomposable to release a nonflammable, inert gas for foaming said second carbonific char into said insulative carbonific char foam, said spumific having a decomposition temperature higher than the decomposition temperatures of said dehydrating agent and said intermediate species.
 4. The coating system of claim 3 wherein said char promoter is a polyhydric alcohol.
 5. The coating system of claim 4 wherein said alcohol is selected from the group consisting of pentaerythritol, dipentaerythritol and mixtures thereof.
 6. The coating system of claim 3 wherein said dehydrating agent is an acid precursor which is thermally decomposable to form said acid catalyst.
 7. The coating system of claim 6 wherein said acid precursor is selected from the group consisting of ammonium phosphate, ammonium polyphosphate, phosphites, organophosphite esters and mixtures thereof.
 8. The coating system of claim 3 wherein said spumific is a dicyandiamide or an azodicarbonamide.
 9. The intumescent coating of claim 3 wherein said binder is selected from the group consisting of an epoxy, a polysulfide, a polysiloxane, a polysilarylene, and mixtures thereof.
 10. The coating system of claim 1 wherein said first coating layer comprises an epoxy resin and a polysulfide.
 11. The coating system of claim 1 further comprising a third coating layer coated on said second film layer and cured thereon to form a third film layer which layer is thermally decomposable upon exposure to said fire conditions to form a third carbonific char which char intumesces to form a second rigid carbonific char foam having a density about twice the density of said insulative carbonific char foam, said insulative carbonific char foam forming under said second rigid carbonific char foam.
 12. The coating system of claim 11 wherein said insulative carbonific char foam has a density of between about 0.01 g/cm³ and less than about 0.5 g/cm³.
 13. The coating system of claim 11 wherein said third coating layer comprises an epoxy resin and a polysulfide.
 14. The coating system of claim 1 further comprising a third coating layer formulated as a blend of said first and said second coating layers, said third coating layer being coated on said second film layer and cured thereon to form a third film layer which layer is thermally decomposable upon exposure to said fire conditions to form a third carbonific char which char intumesces to form a carbonific char foam blend having a first foam component and a second foam component having a density of about half the density of said first foam component, said insulative carbonific char foam forming under said carbonific char foam blend.
 15. The coating system of claim 14 wherein said second foam component has a density of between about 0.01 g/cm³ and less than about 0.5 g/cm³.
 16. A substrate coated according to method for thermally protecting said substrate having a surface exposed to fire conditions comprising the steps of:(a) coating said surface of said substrate with a first coating layer, (b) curing said first coating layer on said surface of said substrate to form a first film layer which layer is thermally decomposable upon exposure to said fire conditions to form a first carbonific char which char intumesces to form a first rigid carbonific char foam (c) coating a second coating layer on said first film layer; and (d) curing said second coating layer on said first coating layer to form a second film layer which layer is thermally decomposable upon exposure to said fire conditions to form a second carbonific char which char intumesces to form an insulative carbonific char foam having a density about half the density height of said first rigid carbonific char foam, said first rigid carbonific char foam forming where said insulative carbonific char foam has broken through to expose said first film layer to said fire conditions: wherein said coating layers have a thickness of less than 50 mils.
 17. A substrate coated according to method for thermally protecting said substrate having a surface exposed to fire conditions comprising the steps of:(a) coating said surface of said substrate with a first coating layer, (b) curing said first coating layer on said surface of said substrate to form a first film layer which layer is thermally decomposable upon exposure to said fire conditions to form a first carbonific char which char intumesces to form a first rigid carbonific char foam; (c) coating a second coating layer on said first film layer; (d) curing said second coating layer on said first coating layer to form a second film layer which layer is thermally decomposable upon exposure to said fire conditions to form a second carbonific char which char intumesces to form an insulative carbonific char foam having a density about half the density height of said first rigid carbonific char foam, said first rigid carbonific char foam forming where said insulative carbonific char foam has broken through to expose said first film layer to said fire condition; (e) coating a third coating layer on said second film layer, and (f) curing said third coating layer on said second coating layer to form a third film layer which layer is thermally decomposable upon exposure to said fire conditions to form a third carbonific char which char intumesces to form a second rigid carbonific char foam having a density about twice the density of said insulative carbonific char foam, said insulative carbonific char foam forming under said second rigid carbonific char foam: wherein said coating layer have a thickness of less than 50 mils.
 18. A substrate coated according to a method for thermally protecting said substrate having a surface exposed to fire conditions comprising the steps of:(a) coating said surface of said substrate with a first coating layer, (b) curing said first coating layer on said surface of said substrate to form a first film layer which layer is thermally decomposable upon exposure to said fire conditions to form a first carbonific char which char intumesces to form a first rigid carbonific char foam; (c) coating a second coating layer on said first film layer; (d) curing said second coating layer on said first coating layer to form a second film layer which layer is thermally decomposable upon exposure to said fire conditions to form a second carbonific char which char intumesces to form an insulative carbonific char foam having a density about half the density height of said first rigid carbonific char foam, said first rigid carbonific char foam forming where said insulative carbonific char foam has broken through to expose said first film layer to said fire conditions; (e) coating a third coating layer on said second film layer, said third coating layer formulated as a blend of said first and said second coating layers; and (f) curing said third coating layer on said second coating layer to form a third film layer which layer is thermally decomposable upon exposure to said fire conditions to form a third carbonific char which char intumesces to form a carbonific char foam blend having a first foam component, and a second foam component having a density about half the foam density of said first foam component, said insulative carbonific char foam forming under said carbonific char foam blend wherein said coating layers have a thickness of less than 50 mils. 